There is at present a strong demand for the improvement of corrosion resistance of a steel sheet forming an automobile body with a view to keeping safety and external appearance of the automobile body for a long period of time. A zinciferous electroplated steel sheet is excellent in corrosion resistance under the effect of a sacrificial protection of corrosion provided by its zinciferous coating. The zinciferous electroplated steel sheet is therefore widely applied as a steel sheet for automobile. Furthermore, a film of a chemically stable corrosion product is formed on the surface of a zinc alloy coating such as an iron-zinc alloy coating of an iron-zinc alloy electroplated steel sheet or a nickel-zinc alloy coating of a nickel-zinc alloy electroplated steel sheet. This film of the corrosion product inhibits a progress of subsequent corrosion of the above-mentioned zinc alloy coating, at a portion thereof, to the surface of which a paint film does not adhere. In addition, the zinc alloy coating, being excellent in alkali resistance, prevents corrosion of the steel sheet caused by alkalinization of water having penetrated through the paint film into the space between the paint film and the zinc alloy coating.
A nickel-zinc alloy electroplated steel sheet has a problem in that, during a progress of corrosion, a content ratio of nickel in the nickel-zinc alloy coating increases along with the decrease in the zinc content in the nickel-zinc alloy coating, thus leading to corrosion of the nickel-zinc alloy electroplated steel sheet. However, an iron-zinc alloy electroplated steel sheet imposes no such problem. Therefore, the iron-zinc alloy electroplated steel sheet has many advantages as a corrosion-resistant electroplated steel sheet.
However, the recent demand for the improvement of corrosion resistance of a steel sheet is becoming remarkably higher than the level of corrosion resistance of the conventional iron-zinc alloy electroplated steel sheet. For the purpose of coping with this increasing demand for a higher corrosion resistance, improvement of corrosion resistance of the coating is tried by adding, to the coating, a metal excellent in corrosion resistance such as chromium, in addition to iron and zinc, and for example, the following electroplated steel sheet is proposed:
An electroplated steel sheet excellent in corrosion resistance disclosed in Japanese Patent Provisional Publication No. 63-243,295 dated Oct. 11, 1988, which has any one alloy coating of the following (a) to (d) (hereinafter referred to as the "prior art"):
(a) an alloy coating, which comprises:
(b) a plurality of alloy coatings, which comprise:
(c) an alloy coating, which comprises:
(d) a plurality of alloy coatings, which comprise:
The above-mentioned prior art has the following problems:
(1) A steel sheet for automobile is required to be excellent not only in corrosion resistance, but also in workability and water-resistant paint adhesivity. However, the electroplated steel sheet having the alloy coating (a) or (c) above of the prior art, i.e., the electroplated steel sheet which has a chromium-zinc alloy coating containing chromium of from over 1 to 70 wt. %, is very poor in workability and water-resistant paint adhesivity. Such an electroplated steel sheet is not therefore suitable as a steel sheet for automobile.
(2) Water-resistant paint adhesivity can be improved by forming an iron-rich iron-zinc alloy coating on the chromium-zinc alloy coating containing chromium of from over 1 to 70 wt. % as in the case of the electroplated steel sheet having the plurality of alloy coatings (b) or (d) of the prior art, under the effect of the iron-zinc alloy coating. However, the iron-zinc alloy coating is susceptible to corrosion. As a result, red rust is produced on the iron-zinc alloy coating, and this impairs formation of a film of a chemically stable corrosion product, thus leading to deterioration of corrosion resistance of the iron-zinc alloy coating. Therefore, formation of the plurality of alloy coatings (b) or (d) above of the prior art on the surface of the steel sheet cannot improve simultaneously both corrosion resistance and water-resistant paint adhesivity.
(3) As described above, it is impossible, in the electroplated steel sheet of the prior art, to satisfy all of workability, corrosion resistance and water-resistant paint adhesivity which a steel sheet for automobile is required to have.
Under such circumstances, there is a demand for the development of an electroplated steel sheet having a plurality of coatings, excellent in workability, corrosion resistance and water-resistant paint adhesivity, but an electroplated steel sheet provided with such properties has not as yet been proposed.